The power factor of an AC electric power system is defined as the ratio of the real power to the apparent power, and is a number between 0 and 1. Real power is the capacity of the circuit for performing work in a particular time. Apparent power is the product of the current and voltage of the circuit. Due to energy stored in the load and returned to the source, or due to a non-linear load that distorts the wave shape of the current drawn from the source, the apparent power can be greater than the real power. Low-power-factor loads increase losses in a power distribution system and result in increased energy costs. Power factor correction (PFC) is a technique of counteracting the undesirable effects of electric loads that create a power factor that is less than 1. Power factor correction attempts to adjust the power factor to unity (1.00).
Many AC-to-DC converter applications require the converter to draw current from the AC line with a high power factor and low harmonic distortion. Most conventional methods to produce a power factor corrected power supply with isolated low voltage DC outputs include cascading converter stages. The term “cascading converter stages” refers to the use of multiple power conversion stages such that the output of one converter stage is connected to the input of the subsequent stage. Each converter stage uses controlled semiconductors such as MOSFETs or IGBTs to control the voltage, current, and/or power at the output and/or input of the converter stage. So, for example, a full-wave passive rectifier bridge is not considered to be a converter stage. While cascaded converter stages may share control circuitry, house-keeping power supplies, or communication with each other, the power semiconductors and energy storage elements that form each converter stage perform a power conversion function that is primarily independent of any other converter stage. Typical examples of converter stages are isolated or non-isolated variants of a buck converter, a boost converter, a buck-boost converter, and a sepic converter.
AC-to-DC power conversion is typically accomplished with cascaded converters instead of with a single-stage converter. For example, many AC-to-DC converters use two primarily independent converter stages: a first converter stage steps up the input rectified sinusoidal voltage to a high-voltage bus, and a second converter stage steps down the high-voltage bus to a low-voltage bus as well as provides isolation. The term “isolation” refers to isolating the input voltage from the output voltage. In particular, isolating means there is no path for DC current between the power supply's input source and its output terminals or load. Isolation is achieved using a power transformer in series with the power flow from input to output. Isolation can be applied to the power converter as a whole, or to individual components within the power converter where the voltage input to the component is isolated from the voltage output from the component.
When the phase of the voltage and current are not aligned due to certain load characteristics, there is a reduction in actual power, which wastes energy. If the phases are perfectly matched, then the power factor is unity. A PFC circuit functions to maximize alignment of the voltage and current. A problem with the PFC circuit is the resulting ripple. The most common meaning of ripple is the small unwanted residual periodic variation of the direct current (DC) output of a power supply which has been derived from an alternating current (AC) source. This ripple is due to incomplete suppression of the alternating waveform within the power supply. Within digital circuits, ripple reduces the threshold at which logic circuits give incorrect outputs and data is corrupted. In the PFC circuit, the input AC voltage is boosted to an output DC voltage. The PFC circuit attempts to maintain a constant DC bus voltage at the output while drawing a current that is always in phase with and at the same frequency as the input AC line voltage. Conversion of AC-to-DC results in ripple, or AC ripple since the ripple is caused be converting the AC line voltage to DC voltage. The ripple generated at the output of the PFC is propagated through a second stage DC-to-DC power converter and experienced as output DC voltage ripple. Loads coupled to the output specify maximum output voltage ripple. It is therefore desirable to design a power supply with output low frequency ripple suppression.
Various approaches have been used to suppress the low frequency output voltage ripple. FIG. 1 illustrates a schematic block diagram of a first approach for suppressing the low frequency output voltage ripple of a power supply. A first stage PFC boost converter converts an input AC voltage, such as 110V or 220V AC line voltage, to a boosted output DC voltage, such as 400V DC, provided as a high voltage bus. The AC ripple on the output 440V DC is at 50-60 Hz. A second stage isolated buck-type converter steps down to an isolated low voltage output, such as 12V DC. A PFC circuit block 2 and a PFC digital controller 4 function as a first stage PFC boost converter. The PFC circuit block 2 is configured to boost the input AC voltage to the boosted output DC voltage. The PFC circuit block 2 is also configured to perform power factor correction under control of the PFC digital controller 4. A DC-DC primary circuit block 6, a transformer 8, a DC-DC secondary circuit block 10, and a DC-DC digital controller 12 function as a second stage isolated buck-type converter. A special type of optical coupler, a linear optical coupler 15, transmits an analog signal from the DC-DC primary circuit block 6 to the DC-DC digital controller 12 across the isolation barrier formed by the transformer 8. The analog signal represents a primary current of the converter in the DC-DC primary circuit block 6, shown as DC-DC primary current in FIG. 1. An isolator circuit block 14 includes an optical coupler and related circuitry for transmitting digital signals from the DC-DC digital controller 12 to the DC-DC primary circuit block 6 across the isolation barrier formed by the transformer 8. The optical coupler in the isolator circuit block 14 is a high speed optical coupler. High speed optical couplers are used to transfer digital signals with high frequencies, typically greater than 10 kHz. Each of the PFC digital controller 4 and the DC-DC digital controller 12 include a UART (Universal Asynchronous Receiver/Transmitter), and an optical coupler 11 enables UART communication between the PFC digital controller 4 and the DC-DC digital controller 12 across the insolation barrier. The optical coupler 11 is either a high speed optical coupler or a low speed optical coupler depending on the baud rate. Low speed optical couplers are used to transfer digital signals with low frequencies, typically equal to or less than 10 kHz. The DC-DC secondary circuit block 10 includes a synchronous rectifier (SR), which is driven by the DC-DC digital controller 12 via a DC-DC SR driver signal.
The second stage isolated buck-type converter has a voltage regulating circuit that includes the DC-DC digital controller 12 and the isolator circuit block 14. The optical coupler circuit in cooperation with the DC-DC digital controller 12 provides a driver signal to the DC-DC primary circuit block 6, shown as DC-DC primary driver in FIG. 1. The DC-DC digital controller 12 is a regulator for the power supply to ensure the output voltage is regulated. The DC-DC digital controller 12 senses the output voltage value and compares the sensed output voltage value to a reference value to generate an error value. The DC-DC primary driver signal is output based on the error value. The DC-DC primary circuit block 6 includes a main switch coupled to the high voltage bus. The DC-DC primary driver signal is used to drive the main switch, the duty cycle of which is adjusted to compensate for any variances in an output voltage Vout. In other embodiments, the DC-DC primary driver signal is simply the error signal, and the DC-DC primary circuit block 6 includes a primary side controller coupled to the main switch. The primary side controller receives the error signal from the isolator circuit block 14 and accordingly adjusts the duty cycle of the main switch to compensate for any variances in an output voltage Vout
Although not explicitly shown in FIG. 1, the circuit can also include an EMI filter, typically coupled between an AC input source and the rest of the converter, to prevent noise from coupling back to the AC source, as well as a full-wave diode rectifier bridge coupled to the EMI filter configured to provide a rectified sinusoidal input voltage to the rest of the converter. The isolated buck-type converter can include various combinations of rectifiers and windings on the secondary side of the transformer to generate an isolated DC output voltage.
The output voltage ripple is suppressed by using a switch current control mode to control loop response for output voltage low frequency ripple suppression where the switch current refers to the DC-DC primary current. FIG. 2 illustrates an exemplary functional block diagram within the DC-DC digital controller 12 of FIG. 1 used to implement the switch current control mode. The switch current control mode uses the input current, DC-DC primary current, as a feedback signal to the control loop. As shown in FIG. 2, the sensed output voltage, Vout Feedback, is compared to a reference output voltage Vout ref, the difference of which is input to a voltage loop control algorithm. The voltage loop control algorithm outputs a reference current Current ref which is compared to the DC-DC primary current, the difference of which is input to a current loop control algorithm. The current loop control algorithm outputs the DC-DC primary driver signal, such as the DC-DC PWM driver signal, that is transmitted to the DC-DC primary circuit block 6 via the isolator circuit 14.
However, using the switch current control mode requires a high frequency analog AC current signal to be transferred from the primary side to the secondary side. Although a low cost transformer can be used for this function, high frequency analog signals can be easily affected by noise. As such, the performance is limited by high frequency analog signal circuit design. Further, processing the analog AC current signal requires an analog-to-digital converter (ADC) in the DC-DC digital controller. Also, the switch current has a relatively high frequency, typically 100 kHz. To recover the analog AC current signal by digital sampling, the sampling frequency needs to be at least double the switch frequency. As such, the control performance is influenced by the digital IC computing speed. The switch current control mode uses an analog AC current signal as a feed forward signal to the DC-DC digital controller on secondary side. However, this approach increases the complexity of the whole design because current needs to added to the control loop. Further, the analog AC current signal delivered from the primary side to the secondary side is not linear, and as such there are significant challenges to obtaining an accurate switch current value.
Instead of the DC-DC digital controller using a switch current control mode, an alternative approach is for the DC-DC digital controller to use input voltage feed forward control for suppressing the low frequency output voltage ripple. FIG. 3 illustrates a schematic block diagram of a second approach for suppressing the low frequency output voltage ripple of a power supply. The power supply shown in FIG. 3 is similar to the power supply of FIG. 1 in that it has a first stage PFC boost converter that converts an input AC voltage to a boosted output DC voltage, and a second stage isolated buck-type converter that steps down the boosted output DC voltage to an isolated low voltage output DC voltage. The first stage PFC boost converter includes a PFC circuit block 22 and a PFC digital controller 24. The second stage isolated buck-type converter includes a DC-DC primary circuit block 26, a transformer 28, a DC-DC secondary circuit block 30, and a DC-DC digital controller 32. An isolator circuit block 34 includes a high speed optical coupler and related circuitry for transmitting a DC-DC primary driver signal from the DC-DC digital controller 32 to the DC-DC primary circuit block 26. Each of the PFC digital controller 24 and the DC-DC digital controller 32 include a UART, and an optical coupler 31 enables UART communication between the PFC digital controller 24 and the DC-DC digital controller 32 across the insolation barrier. The optical coupler 31 is either a high speed optical coupler or a low speed optical coupler depending on the baud rate. The first stage PFC boost converter and the second stage isolated buck-type converter function similarly as in FIG. 1 to regulate the output voltage. However, instead of using switch current for low frequency output voltage ripple suppression, the circuit of FIG. 3 transfers an analog signal representative of the high voltage bus, which includes the AC ripple component, at the output of the PFC circuit block 2. A linear optical coupler 36 is used to sense the AC ripple component of the high voltage bus, generate a corresponding analog signal and optically transmit the analog signal to the DC-DC digital controller 32. The voltage at the high voltage bus is a combination of a DC component having a voltage level of about 400V DC and the AC ripple component with a peak-to-peak voltage of about 20V and a frequency of 100-120 Hz, which is double that of the AC line frequency. The AC ripple on the output from the PFC circuit block 22 is very small component, e.g. 10-20V or 2-5%, of the overall 400V signal at the high voltage bus. In order for the AC ripple to be suppressed, the size of the AC ripple needs to be quantified and conveyed to the DC-DC digital controller 32. A linear optical coupler is required to accurately convey this small relative amount of the AC ripple. In order to transmit an analog signal from the primary side to the secondary side, a linear optical coupler is necessary because the linear optical coupler provides a linear relationship between the input signal and the output signal. If instead an optical coupler of the type that transmits digital signals is used, it may not be possible to differentiate the AC ripple component from the entire transmitted signal.
FIG. 4 illustrates an exemplary functional block diagram within the DC-DC digital controller 32 of FIG. 3 used to implement the input voltage feed forward control. The input voltage feed forward control uses the AC portion of the PFC Vbus transmitted as the analog signal via the linear optical coupler 36 as a feedback signal to the control loop. As shown in FIG. 4, the sensed output voltage, Vout Feedback, is compared to a reference output voltage Vout ref, the difference of which added to a digital conversion of the analog signal and input to a voltage loop control algorithm. The voltage loop control algorithm outputs the DC-DC primary driver signal, such as the DC-DC PWM driver signal, that is transmitted to the DC-DC primary circuit block 26 via the isolator circuit 34.
A linear optical coupler configured to optically transmit an analog signal is considerably more expensive than an optical coupler of the type included in the isolator circuit block 34, configured to accurately transmit a digital signal. The analog signal optically transmitted by a linear optical coupler represents an amplitude compensation corresponding to the amplitude of the AC ripple. However, AC ripple includes both the amplitude component and a phase component, where the phase component is the difference between the AC voltage and AC current waveforms at the high speed bus. The phase component is not included in the optically transmitted analog signal. Additionally, the control loop for the second stage isolated buck-type converter is a digital control loop. Therefore, the analog signal optically transmitted by the linear optical coupler 36 can not be input to the DC-DC digital controller via a general purpose input/output (GPIO) interface. Instead, the DC-DC-digital controller must include an analog-to-digital converter (ADC) channel for receiving the analog signal.